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630                Various Medium-speed Engines.

The side lever marine engine F, the first form considerably
adopted on steamboats, was but a beam engine doubled upon
itself so as to save room, a is the paddle-shaft, b the steam
cylinder, c the beam or ' side lever/ and d the air purnp.

The Direct-acting Engine is shewn in various forms in
diagrams G to R, Fig. 626. G is a horizontal factory engine, with
condenser a behind a cylinder <, so that the air pump may be
worked in a simple manner by projecting the piston-rod back-
ward. By dispensing with the beam very considerable friction at
the trunnion bearing is avoided, caused as such friction was by
both load and resistance, or double the piston load. In the
horizontal engine there is, however, some additional frictional
loss, due to weight of parts and thrust of connecting-rod, while in
the vertical engine, although the former is eliminated, the latter
still remains.

The diagonal paddle engine at H, like other marine engines,
is designed to save room. Whenever paddle propulsion is em-
ployed, these engines are now chosen for the purpose. The
condenser and air pump are placed within the (triangle.7 j is a
form of factory engine seldom employed, but given as an example
of,a vertical engine with cylinder at bottom and crank overhead;
the slide valve replaces the four drop valves of Fig. 629, being
worked by eccentric, from the crank shaft.

Two other paddle engines are shewn at -Q and M. Q is the
oscillating engine, ekceedingly simple so far as the main mechanism
is concerned, dispensing with a connecting rod; but the valve gear
is more complicated than with fixed cylinders. The steeple
engine (M) was introduced to save head room in shallow boats.
Two piston rods are employed, and the paddle shaft is placed
between crosshead and cylinder; the connecting rod is said to
be '.returned.' The principal objections to this design are the
difficulty of staying the slide bars, and of keeping two parallel
glands steam tight.

The Penn trunk engine (N) and Maudslay return-connecting
rod engine (p) are examples of early screw engines. Being both
placed athwart the ship, they must be shortened in length as
much as possible. Penn got rid of piston rod length by using a
trunk piston and driving the air pump by a rod connected directly